94. Binary Tree Inorder Traversa

递归！！！容易理解！！！

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> result;
        traverse(root, result);
        return result;
        
    }
private:
    void traverse(TreeNode* root, vector<int> &ret){
        if(root){
            traverse(root->left, ret);
            ret.push_back(root->val);
            traverse(root->right, ret);
        }
    }
};

迭代：不太好理解：